The organic electroluminescence phenomenon was first reported in the early 1960s. Pope and other people observed blue light when a high voltage of 400V was imposed to both sides of a single crystal of anthracene (see M. Pope, H. Kallmann and P. Magnante, J. Chem. Phys., 1963, 38, 2042). However, because the growth of the single crystal was so difficult and the driving voltage was so high, such process can almost be used in practice. Thus the development of organic electroluminescence remained stagnant.
Until 1987, by ultra-thin-film technology, C. W. Tang, hired by Kokak, a company in the United States, and other people used diamine derivatives, which had a better hole transporting effect, as the hole-transporting layer, 8-hydroxyquinoline aluminum (Alq3) as emitting layer, transparent indium tin oxide (ITO) film as anode, and magnesium-silver alloy as cathode, to obtain a green light of brightness up to 1000 Cd/m2 under a driving voltage of 10V. The efficiency of the device was 1.5 lm/W, and the life was above 100 hours (see C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett., 1987, 51, 913). This breakthrough made the studies on organic electroluminescence being carried out rapidly and intensively in the world.
C. W. Tang first discovered that Alq3 had good electroluminescent properties, 8-hydroxyquinoline and its derivatives were then used together with Al3+, Zn2+, Ga3+, Be2+, and the like to synthesize a series of complexes as electroluminescent luminescent material, and most of them emitted yellow-green light, and some emitted blue light (U.S. Pat. No. 4,720,432; U.S. Pat. No. 4,539,507; U.S. Pat. No. 5,151,629; Y. Hamada et al., Jpn. Y. Appl. Phys., Part 2, 1992, 32, L514; M. Matsumura et al., Jpn. J. Appl. Phys., 1996, 35, 5357; P. E. Burrows et al., J. Appl. Phys., 1996, 79, 7991). As disclosed in USP No. 5,432,014, Sano et al., hired by Sanyo, accompany in Japan, use Schiff base—zinc complexes as the emitting layer to prepare a blue-ray device with good performance. It is notable that 10-hydroxy-benzo-quinoline synthesized by Hamada et al., Sanyo Japan, has a better electroluminescent performance than Alq3 (Y. Hamada et al., Chem. Lett., 1993, 905).
In 1996, Hamada et al. used 2-(2-hydroxy-phenyl)-benzothiazole chelated with zinc (Zn(BTZ)2) as the emitting layer and the electron-transporting layer to prepare a device having a structure of indium tin oxide (ITO) (anode)/aromatic diamine derivative (TPD) (the hole-transporting layer)/Zn(BTZ)2 (emitting layer)/MgIn (anode) by vacuum deposition, and emitting a green-white electroluminescence. This electroluminescence spectrum is the same as the photoluminescence spectrum; both at 486 nm and 524 nm, and having the half width of 157 nm, and the device accordingly emitted visible green-white light. The chromaticity coordinate was (0.246, 0.363), the maximum brightness may be 10190 cd/m2 under a driving voltage of 8V, and the lumens efficacy was 0.89 lm/W (see Yuji Hamada, Takeshi Sano, Hiroyuki Fujii, et al, white-light-emitting materials for organic electroluminescent devices, Jpn. J. Appl. Phys., 1996, 35, 1339-1341).
The continuous development of organic electroluminescent material greatly promotes the progress of electroluminescent devices and makes the devices approximately into practical use. In recent years, a lot of money and effort are expended on the development of new materials, and many kinds of material of excellent properties have brought some breakthrough to the electroluminescent.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.